Micro Sensor Monitors Properties of Liquids

The Vienna Technical University is developing sensors that can measure the quality of lubricants. The devices use a microchip aluminium nitride cantilever that vibrates. The mechanical resistance that the oil -- or any other liquid -- imposes to the cantilever is measured and provides insights to the oil viscosity.

Today, measuring the quality of a lubricant requires quite an effort: It is necessary to take a sample of the liquid, send it to a laboratory, and wait for the results. If large machines stand still for days for this reason, it becomes rather expensive. "Therefore, our goal is developing a micro sensor that can measure oil density and viscosity directly in the machine. Users thus are informed at any time about the aging status of the lubricant", says professor Ulrich Schmid from the institute for sensor and actuator systems of the Vienna Technical University. For this project, his team maintains a tight collaboration with the excellence center of tribology ACT2 in Wiener Neustadt.

The core element of the sensor is a tiny cantilever of silicon which is coated by aluminium nitride; the micro structure measures 2,500 x 1,300. The resonance and vibration properties of the cantilever depend greatly of the liquid surrounding it; these vibrations can be measured very precisely.

The measurement principle is similar to the one used in atomic force microscopes. In both cases, the vibration of a microstructure provides the measurement result. However, while in an atomic force microscope the vibrations are measured with a laser beam, such a principle would be much too complex for testing lubricants. Therefore, the Austrian researchers developed a purely electronic solution. The aluminium nitride coating has a piezoelectric behavior; it can be excited by adding electric alternating voltages. If the cantilever vibrates and experiences deformations, free charge carriers are generated at its surface, and its conductivity changes. Vibration generation, as well as readout, is done electronically.

The decisive idea was the development of a specific electronic circuit: The measurement signal is fed back to the input signal. This feedback loop enables measuring the resonance properties of the cantilever very precisely. It was necessary to integrate very effective noise cancelling to selectively amplify only the desired signal.

A benefit of this measurement method is that it can easily be integrated into systems. Plus, aluminium nitride is a material that can be handled without problems by just about any of today's chip factories. The electronic circuitry can be scaled down to chip level easily, and there is no big effort required to excite the vibrations. For this reason, the Vienna researchers believe that it very easily could be possible to integrate the measurement system into a multifunctional device that also measures further parameters in the machine.

I believe that this chip could help to safe much money and protect the environment. In many machines oil is changed too early in order to be on the safe side and to avoid damages. Maybe this chip is also interesting for cars in order to signal drivers when the next oil change is really necessary.